1. Field of the Invention
The present invention relates generally to an exhaust gas characteristic detecting apparatus for an internal combustion engine, and more particularly, to an exhaust gas characteristic detecting apparatus for use with an internal combustion engine which has a double-structure exhaust passage comprised of an inner exhaust passage and an outer exhaust passage for detecting the characteristic of exhaust gases which flow through the inner exhaust passage.
2. Description of the Prior Art
A conventional internal combustion engine having a double-structure exhaust passage is known, for example, in Laid-open Japanese Patent Application No. 7-332074. An exhaust system of this internal combustion engine comprises a catalyzer arranged in an intermediate portion of the exhaust system, and an inner exhaust passage and an outer exhaust passage which are branched off at a location downstream of the catalyzer and joined at a location downstream of the branch. The outer exhaust passage is arranged to completely surround the inner exhaust passage. Also, the inner exhaust passage is filled with a hydrocarbon (HC) adsorbent for adsorbing hydrocarbons and moisture in exhaust gases. The inner exhaust passage is joined to the outer exhaust passage at a location immediately downstream of the HC adsorbent. Further, a flow path switching valve is provided at the joint for switching a flow path of exhaust gases from the catalyzer to the outer exhaust passage or to the inner exhaust passage. In this internal combustion engine, the flow path of exhaust gases is switched by the flow path switching valve to the inner exhaust passage immediately after the engine is started. Hydrocarbons included in the exhaust gases immediately after the start cannot be purified by the catalyzer, since it has not been activated due to a low temperature immediately after the start, and therefore are adsorbed by the HC adsorbent when they pass therethrough. Subsequently, as the catalyzer is heated by the heat of the exhaust gases after the start of the engine and eventually activated, the exhaust gas flow path is switched to the outer exhaust passage by the flow path switching valve. This causes the exhaust gases to flow through the outer exhaust passage, so that the HC adsorbent is heated by heat exchange with the exhaust gases to desorb the adsorbed hydrocarbons. The desorbed hydrocarbons are recirculated to the catalyzer for purification.
In addition, since the HC adsorbent adsorbs hydrocarbons and moisture in exhaust gases, a humidity sensor may be provided, for example, at a location downstream of the HC adsorbent such that a deterioration of the HC adsorbent can be determined by detecting the humidity of exhaust gases which have passed through the HC adsorbent.
Further, a structure for attaching a sensor to an exhaust pipe is known, for example, in Laid-open Japanese Patent Application No. 7-332074. This sensor, which is an oxygen concentration sensor for detecting an oxygen concentration, is attached at a location downstream of a catalyst in an exhaust pipe with a sensing unit at a leading end thereof protruding into the exhaust pipe. The exhaust pipe is also provided with a guide pipe for guiding exhaust gases, after passing through the catalyst, to the sensing unit of the oxygen concentration sensor. This guide pipe is formed along the exhaust pipe and tapered toward the sensing unit of the oxygen concentration sensor.
When the aforementioned deterioration determining technique is applied to the conventional internal combustion engine, a humidity sensor must be arranged at a location downstream of an adsorbent in order to detect the humidity in exhaust gases which have passed through the adsorbent. In this event, a portion immediately downstream of the adsorbent is used as a joint at which both exhaust passages are joined together and in which a flow path switching valve is also arranged. Therefore, if an attachment structure similar to that for the oxygen concentration sensor is simply applied to attach the humidity sensor to the exhaust system, a guide pipe and the humidity sensor must be arranged at locations downstream of the joint. As a result, exhaust gases from the outer exhaust passage as well as exhaust gases from the inner exhaust passage pass through the humidity sensor, so that the humidity sensor is exposed to the exhaust gases at all times irrespective of whether or not the humidity need be detected, resulting in a higher susceptibility to corrosion and so on and a shorter lifetime.
The present invention has been made to solve the problems as mentioned above, and its object is to provide an exhaust gas characteristic detecting apparatus for an internal combustion engine which is capable of reliably detecting the characteristic of exhaust gases passing through an inner exhaust passage out of inner and outer exhaust passages in a double-structure exhaust passage, and capable of extending the lifetime of a sensor.
To achieve the above object, the present invention provides an exhaust gas characteristic detecting apparatus for an internal combustion engine which has an inner exhaust passage and an outer exhaust passage arranged to surround the outside of the inner exhaust passage such that exhaust gases are discharged through one of the inner and outer exhaust passages. The exhaust gas characteristic detecting apparatus is adapted to detect a characteristic of exhaust gases passing through the inner exhaust passage, and includes a guide pipe extending across the outer exhaust passage and having an exhaust gas introducing port facing the inner exhaust passage for introducing exhaust gases flowing into the inner exhaust passage into the guide pipe, and a sensor having a sensing unit for detecting the characteristic (for example, humidity) of exhaust gases and arranged such that the sensing unit is positioned in the guide pipe.
According to the foregoing exhaust gas characteristic detecting apparatus for an internal combustion engine, exhaust gases flowing into the inner exhaust passage is introduced into the guide pipe through the exhaust gas introducing port. Then, the characteristic of the introduced exhaust gases is detected by the sensing unit of the sensor which is positioned in the guide pipe. It is therefore possible to reliably detect the characteristic of the exhaust gases flowing into the inner exhaust passage. Also, since the guide pipe extends across the outer exhaust passage, the sensing unit is protected by the guide pipe when exhaust gases flow through the outer exhaust passage and therefore is not exposed to the exhaust gases. As a result, the sensing unit of the sensor is exposed to the exhaust gases for a shorter time than before, so that the sensor is less susceptible to corrosion and so on and can extend the lifetime.
Preferably, in an embodiment of the exhaust gas characteristic detecting apparatus for an internal combustion engine, the inner exhaust passage includes an adsorbent arranged therein for temporarily adsorbing unburnt components and moisture in exhaust gases flowing into the inner exhaust passage and desorbing the once adsorbed unburnt components and moisture, the guide pipe is arranged at a location downstream of the adsorbent in the inner exhaust passage, and the sensor includes a humidity sensor for detecting a humidity of exhaust gases which have passed through the adsorbent.
According to this embodiment, since the humidity of exhaust gases which have passed through the adsorbent can be detected by the humidity sensor, the moisture adsorbing capability of the adsorbent, for example, can be determined by a detection signal indicative of the humidity detected by the humidity sensor to make a deterioration determination for the adsorbent in consequence.